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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Interacting with C</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="containers.html" title="Chapter 9.  Containers" /><link rel="prev" href="unordered_associative.html" title="Unordered Associative" /><link rel="next" href="iterators.html" title="Chapter 10.  Iterators" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Interacting with C</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="unordered_associative.html">Prev</a> </td><th width="60%" align="center">Chapter 9. 
  Containers
  
</th><td width="20%" align="right"> <a accesskey="n" href="iterators.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.containers.c"></a>Interacting with C</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="containers.c.vs_array"></a>Containers vs. Arrays</h3></div></div></div><p>
     You're writing some code and can't decide whether to use builtin
     arrays or some kind of container.  There are compelling reasons
     to use one of the container classes, but you're afraid that
     you'll eventually run into difficulties, change everything back
     to arrays, and then have to change all the code that uses those
     data types to keep up with the change.
   </p><p>
     If your code makes use of the standard algorithms, this isn't as
     scary as it sounds.  The algorithms don't know, nor care, about
     the kind of <span class="quote">“<span class="quote">container</span>”</span> on which they work, since
     the algorithms are only given endpoints to work with.  For the
     container classes, these are iterators (usually
     <code class="code">begin()</code> and <code class="code">end()</code>, but not always).
     For builtin arrays, these are the address of the first element
     and the <a class="link" href="iterators.html#iterators.predefined.end" title="One Past the End">past-the-end</a> element.
   </p><p>
     Some very simple wrapper functions can hide all of that from the
     rest of the code.  For example, a pair of functions called
     <code class="code">beginof</code> can be written, one that takes an array,
     another that takes a vector.  The first returns a pointer to the
     first element, and the second returns the vector's
     <code class="code">begin()</code> iterator.
   </p><p>
     The functions should be made template functions, and should also
     be declared inline.  As pointed out in the comments in the code
     below, this can lead to <code class="code">beginof</code> being optimized out
     of existence, so you pay absolutely nothing in terms of increased
     code size or execution time.
   </p><p>
     The result is that if all your algorithm calls look like
   </p><pre class="programlisting">
   std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);
   </pre><p>
     then the type of foo can change from an array of ints to a vector
     of ints to a deque of ints and back again, without ever changing
     any client code.
   </p><pre class="programlisting">
// beginof
template&lt;typename T&gt;
  inline typename vector&lt;T&gt;::iterator
  beginof(vector&lt;T&gt; &amp;v)
  { return v.begin(); }

template&lt;typename T, unsigned int sz&gt;
  inline T*
  beginof(T (&amp;array)[sz]) { return array; }

// endof
template&lt;typename T&gt;
  inline typename vector&lt;T&gt;::iterator
  endof(vector&lt;T&gt; &amp;v)
  { return v.end(); }

template&lt;typename T, unsigned int sz&gt;
  inline T*
  endof(T (&amp;array)[sz]) { return array + sz; }

// lengthof
template&lt;typename T&gt;
  inline typename vector&lt;T&gt;::size_type
  lengthof(vector&lt;T&gt; &amp;v)
  { return v.size(); }

template&lt;typename T, unsigned int sz&gt;
  inline unsigned int
  lengthof(T (&amp;)[sz]) { return sz; }
</pre><p>
     Astute readers will notice two things at once: first, that the
     container class is still a <code class="code">vector&lt;T&gt;</code> instead
     of a more general <code class="code">Container&lt;T&gt;</code>.  This would
     mean that three functions for <code class="code">deque</code> would have to be
     added, another three for <code class="code">list</code>, and so on.  This is
     due to problems with getting template resolution correct; I find
     it easier just to give the extra three lines and avoid confusion.
   </p><p>
     Second, the line
   </p><pre class="programlisting">
    inline unsigned int lengthof (T (&amp;)[sz]) { return sz; }
   </pre><p>
     looks just weird!  Hint:  unused parameters can be left nameless.
   </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="unordered_associative.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="containers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="iterators.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Unordered Associative </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 10. 
  Iterators
  
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